Field of the Invention
The present invention concerns a method for magnetic resonance imaging with a measurement sequence of the “free precession of transverse magnetization in the steady state”-type—an SSFP measurement sequence—and a magnetic resonance system for implementing such a method. In particular, the invention concerns measurement sequences that include the radiation of a multidimensional, spatially selective radio-frequency pulse.
Description of the Prior Art
Magnetic resonance (MR) tomography is an imaging method that enables the acquisition of two-dimensional or three-dimensional image data sets that can depict structures inside an examined person with high resolution. In MR imaging, the magnetic moments of protons in an examination subject are aligned in a basic magnetic field so that a macroscopic magnetization arises along a longitudinal direction. This is subsequently deflected (excitation) out of the steady state parallel to the basic magnetic field (longitudinal magnetization) by the radiation of radio-frequency (RF) pulses. A transverse magnetization is thereby generated. Special RF transmission coils of an MR system are typically used for radiation.
The decay of the transverse magnetization back into the steady state or the magnetization dynamic is subsequently detected (imaging) by means of one or more RF reception coils of the MR system as MR data. A spatial coding of the acquired MR data is achieved by the application of different magnetic field gradients (for slice selection, phase coding or readout coding). The transverse magnetization in a defined slice is excited out of the steady state with conventional RF pulses via the simultaneous switching of the slice selection gradient field. The excitation is typically not limited within the slice, and therefore can be designated as one-dimensionally (1D) spatially selective. A targeted dephasing/rephasing of the transverse magnetization to obtain a signal known as a gradient echo by the application of the phase coding and readout gradient fields. The acquired MR signals i.e, the detected (and therefore spatially resolved) MR data initially exist as raw data in the spatial frequency domain (called k-space), and can be transformed into the spatial domain (image space) by subsequent Fourier transformation. K-space can be scanned (data entered therein) along a well-defined k-space trajectory by means of the gradient fields.
A need frequently exists for measurement sequences that have a spatially limited measurement region. This can be the case when a relevant region (region of interest, ROI) is relatively limited, for instance due to anatomical conditions of an examined person.
Techniques are known for this that discard MR data outside of the measurement region based on an image segmentation of the MR images. However, the unnecessary acquisition of MR signals outside of the measurement region can unnecessarily extend the entire time period that is required to implement the measurement sequence (measurement duration).
Further-developed RF pulses—for instance for multidimensional, spatially selective excitation—have recently been designed, for example attempts to shorten the measurement duration. Such multidimensional, spatially selective RF pulses can use special k-space trajectories to excite the transverse magnetization. Clearly spatially defined (and limited, for example) regions in two dimensions (2d) or three dimensions (3d) can thereby be excited with a specific, well-defined phase of the transverse magnetization. For example, gradient fields along multiple axes can be used for this. It is also possible to provide a special amplitude modulation of the multidimensional, spatially selective RF pulse. For instance, examples of such pulses are known from “Two-Dimensional Spatially-Selective RF Excitation Pulses in Echo-Planar Imaging” by S. Riesenberg et al. in Mag. Reson. Med. 47 (2002) 1186-1193.
In comparison to conventional RF pulses (for example 1D slice-selective RF pulses), such multidimensional, spatially selective RF pulses can require a longer time period for implementation. However, the number of scanning points in the imaging can be limited, and the overall measurement duration thus can be reduced.
However, physiological restrictions can limit or preclude the use of such multidimensional, spatially selective RF pulses. For instance, in cardiac imaging the time period required to implement the RF pulse can be severely limited due to heart movement, etc. Typical acceptable time durations can be in the range of a few milliseconds, for instance in the range of 1 ms. Typical known multidimensional RF pulses can be used only as limited to such short time scales. This particularly applies to fast measurement sequences for MR imaging that utilize a free precession of the transverse magnetization in the steady state. Such measurement sequences are also designated as SSFP (Steady State Fast Precession) measurement sequences. In general, SSFP measurement sequences can be categorized as a gradient echo sequence with a repetition time TR that is shorter than the T1 relaxation time.